In such a printer it is known to provide the rotating screen with an internal mobile wiper which, under its own weight supplemented by magnetic attraction, bears upon the lowest point or nadir of the screen circumference to establish a line of contact with the substrate. The attractive force is generally provided by a bank of magnets disposed in or below the worktable, the upright cores of those magnets having aligned pole faces which are flush with the upper table surface and are bisected by a common midplane paralleling the axis of rotation.
Curved blades or slot wipers, weighted by masses of magnetizable material such as soft-iron plates or bars, may be used in such a printer for holding down the screen and forcing the dyestuff through its perforations. Frequently, however, the wiper is designed as a magnetizable roller which rotates on the inner peripheral surface of the turning screen. Conventional wisdom places the screen axis in the vertical midplane of the pole faces in the expectation that this will hold the roller directly above the nadir for the production of sharp prints. In practice, however, it has been found that a roller so positioned has a tendency to wander over the area of the pole faces, in the direction of screen rotation, so that its own centerline no longer lies in the axial vertical plane of the screen and may even become oriented skew to the line of contact. These excursions of the roller, taking place during operation, may cause varying accumulations of dyestuff above the line of contact and, since the dyestuff no longer penetrates the screen at its nadir, result in a blurred and irregular print. Presumably, the described phenomenon is essentially due to the fact that the magnetic force acting upon the roller--or, for that matter, on any other sort of wiper--does not vary significantly over the pole-face area and thus does not constrain the wiper to remain centered with reference thereto. A larger gradient of that force, with a greater restraining effect, exists at the downstream boundary of the area toward which the roller tends to migrate. I have found that the extent of this migration depends mainly on the ratio between the roller diameter and the width of the pole-face area, this ratio being generally a fixed parameter, and on the variable thickness of the substrate to be imprinted.